(a) Field of the Invention
The present invention discloses an organic light-emitting diode device and a manufacturing method thereof. By means of the design of the light emitting layer structure, the device emits visible light with a lower color temperature at low voltages, and emits visible light with a higher color temperature at high voltages. By adjusting the input voltage, the device is capable to emit white light or other color lights with desired color temperature.
(b) Description of the Prior Art
An organic electro-luminescence display or an organic EL display is also referred to as an organic light emitting diode (OLED). C. W. Tang and S. A. Vanslyke et al. of Eastman Kodak Company used a vacuum evaporation method to make it in 1987 first. The hole transporting material and electron transporting material were deposited on transparent indium tin oxide glass, and then a metal electrode was vapor-deposited thereon to form the self-luminescent OLED device. Due to high brightness, fast response speed, light weight, compactness, true color, no difference in viewing angles, no need of LCD type backlight plates as well as a saving in light sources and low power consumption, it has become a new generation display.
Referring to FIG. 1, there is a cross-sectional view showing a structure of an OLED device according to the prior art. The OLED device structure sequentially comprises, from bottom to top, a transparent substrate 11, a transparent anode (ITO) 12, a hole transporting layer (HTL) 13, an organic light emitting layer (EL) 14, an electron transporting layer (ETL) 15, an electron injection layer (EIL) 16, and a metal cathode 17. When a forward bias is applied, holes 131 are injected from the anode 12 and electrons 151 are injected from the cathode 17. Due to the potential difference incurred from the external electrical field, the electrons 151 and the holes 131 will move in the thin film and recombine with each other in the organic light emitting layer 14. A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the organic emitting layer 14 to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence.
Referring to FIG. 2, there is a cross-sectional view showing a structure of another OLED device according to the prior art. The OLED device was described by C. W. Tang in U.S. Pat. No. 4,356,429 (Eastman Kodak Company, 1982). In the invention, the OLED device structure sequentially comprises, from bottom to top, a transparent substrate 21, a transparent anode 22, a hole injection layer 23, a light emitting layer 24, and a metal cathode 25. When a forward bias is applied, holes are injected from the anode 22 and electrons are injected from the cathode 25. Due to the potential difference incurred from the external electrical field, the electrons and holes will move in the thin film and recombine with each other in the light emitting layer 24. A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the light emitting layer 24 to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence.
Referring to FIG. 3, there is also a cross-sectional view showing a structure of an OLED device of the prior art. The OLED device was proposed by C. W. Tang in U.S. Pat. No. 4,720,432 (Eastman Kodak Company, 1988). In the invention, the OLED device structure sequentially comprises, from. bottom to top, a transparent substrate 31, a transparent anode 32, a hole injection layer 33, a light emitting layer 34 having an electron transporting function, and a metal cathode 35. When a forward bias is applied, holes are injected from the anode 32 and electrons are injected from the cathode 35. Due to the potential difference incurred from the external electrical field, electrons and holes will move in the thin film and recombine with each other in the light emitting layer 34. A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the light emitting layer 34 to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence.
Referring to FIG. 4, therein illustrated is a doped type OLED device proposed by C. W. Tang et al. in Journal of Applied Physics, vol. 65, p. 3610 (1989). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate 41, a transparent anode 42, a hole transporting layer 43, a single component light emitting layer 44, a dye-doped light emitting layer 45, a single component light emitting layer 46, and a metal cathode 47, which also can give organic electroluminescence.
Referring to FIG. 5, therein illustrated is a doped type OLED device proposed by C. H. Chen et al. in Applied Physics Letters, vol. 85, p. 3301 (2004). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate 51, a transparent anode 52, a hole injection layer 53, a hole transporting layer 54, a dye-doped light emitting layer 55, an electron transporting layer 56, an electron injection layer 57, and a metal cathode 58, which can give organic electroluminescence.
An OLED can emit light of different wavelengths based on the luminescent materials used. White light can be produced by mixing complementary lights. The luminescent materials can be arranged in different layers or can be deposited in the same light emitting layer. Referring to FIG. 6, therein illustrated is a white light OLED device with a single light emitting layer proposed by the present inventors in Applied Physics Letters, vol. 88, p. 193501 (2006). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate 61, a transparent anode 62, a hole transporting layer 63, a doped type white light emitting layer 64, an electron transporting layer 65, an electron injection layer 66, and a metal cathode 67. The white light emitting layer 64 can be composed of a blue light emitting host doped with red light emitting dyes, or further, a blue light emitting host doped with green and red light emitting dyes, which emits white light related to organic electroluminescence.
In addition to utilizing complementary lights generated by organic electroluminescence to produce white light, organic electroluminescence and photoluminescence can be used to produce white light. Referring to FIG. 7, therein illustrated is a light source device combining an organic layer and a photoluminescent layer proposed by A. R. Duggal et al. in U.S. Pat. No. 6,847,162 (General Electric (GE) Company). The light source 71 sequentially comprises, from bottom to top, an OLED device 72 which emits blue light, a transparent substrate 73, and a photoluminescent layer 74. The photoluminescent layer 74 absorbs the blue light emitted by the OLED device 72 and emits yellow light having lower energy. The light source mixes blue light and yellow light to produce white light.
The color temperature of white light mixed from monochromatic lights of blue, green, red, and the like can be changed by tuning the intensity of each monochromatic light. Referring to FIG. 8, there is a schematic view showing a structure of a color tunable organic electroluminescent light source device proposed by A. R. Duggal et al. in U.S. Pat. No. 6,661,029 (General Electric (GE) Company). The light emitting device 81 comprises an integrated controller 82, an red light emitting OLED 83, an green light emitting OLED 84 and an blue light emitting OLED 85. The multiple monochromatic light OLEDs are connected with circuits to form light emitting device sets 86, 87 and 88 having larger light emitting area, respectively. The device further comprises a power source 89 that is electrically connected together to the integrated controller 82 and the OLEDs 83, 84 and 85. The intensity of each monochromatic light can be tuned by the integrated controller 82 to change the color temperature of the mixed white light.
As a result of a variety of extensive and intensive studies and discussions, the inventors herein propose an organic light-emitting diode device and a manufacturing method thereof based on their research for many years and plenty of practical experience. By means of the design of the light emitting layer structure of the device, the device is capable to emit white light or other color lights with desired color temperature without additional circuit control and only by adjusting the input voltage. The present invention has been accomplished based on these findings.